This study examined undergraduate multilingual writers' disciplinary writing practices by drawing on the academic literacies framework (Lea and Street, 1998), which conceptualizes student writing at the level of social identities, epistemological varieties in disciplines, and institutional power relations rather than at the skill-based level. Existing studies on multilingual undergraduate students have focused on their initial writing experiences in their first year of university study. Little attention has been paid to what happens in their later undergraduate years. This study addressed that gap by focusing on senior-level undergraduate multilingual writers' writing experiences in disciplinary courses. It examined the possible roles that knowledge and identity played in their disciplinary writing and learning in order to shed light on the nature of these undergraduate students' transitions into disciplinary writing after prior exposure to English as a Second Language (ESL) and First Year Writing (FYW) courses. This multiple-case study involved five undergraduate multilingual writers who were in different disciplinary fields. Drawing on multiple sources of data, including writing samples, stimulated recall protocols, interviews with student writers and professors, and related course materials, this study explored the faculty expectations for student writing at the undergraduate level and the composition practices that the participants used (consciously or unconsciously) to meet these expectations. The findings revealed that disciplinary knowledge played a critical role in demonstrating and assessing writing competence, and issues of writer identity emerged, as the participants were often concerned with how their construction of disciplinary knowledge as a novice/student would be perceived by the professor/expert. Their use of composition practices revealed their intentionality as writers and the attempts that they made to delimit their shortcomings in terms of disciplinary content and writing knowledge to meet faculty expectations. Specifically, the participants' disciplinary writing practices were influenced by their authorial identity, task representation, transfer of learning, and the transparency of faculty expectations. The examination of the participants' disciplinary writing practices suggested that disciplinary knowledge and identities have a critical impact on how student writers develop their ideas and construct authorial presence in writing, and that writing in the disciplines involves the complex process of engaging disciplinary knowledge and practices while developing disciplinary identities. The investigation of faculty expectations revealed that disciplinary faculty often incorrectly assumed that students would transfer knowledge learned about writing in generic academic writing courses to the more nuanced writing expected in the disciplines. These findings shed light on the critical roles that faculty members can play to provide support for students' engagement with disciplinary practices, through which disciplinary identity is developed for successful disciplinary learning and writing. Overall, this study offered a critical lens to examine undergraduate student writing by exploring their writing process and portraying their intentionality as writers rather than viewing issues in their writing as simply "problems" or "mistakes," a shift in focus that can reframe and broaden the ways researchers approach the study of students' writing practices and increase instructors' awareness of the complexities of writing development. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]